Methods for the Characterization and Measurement of the Gain Fluctuations of Cryogenic Amplifiers

Since ALMA instrument will have low noise receivers with very large instantaneous bandwidth, gain stability should be carefully specified to avoid limiting the sensitivity of the total system. This memorandum reviews the methods for the characterization of gain stability in the time and frequency domain and presents examples of experimental setups used for its measurement in the case of cryogenic amplifiers of the type used in the IF of ALMA as well as its practical limitations. Special attention is paid to some important but not widely known effects, as the dependence of gain fluctuation with bias and its variation across the band of the amplifier. Finally, the compliance of one prototype 4-12 GHz InP cryogenic IF amplifier with the requirements of ALMA project is discussed. Methods for the Characterization and Measurement of the Gain Fluctuations of Cryogenic Amplifiers 1. ALMA Specification: time and frequency domain The specification of gain stability for the cryogenic IF amplifier of an ALMA receiver is derived from the total System Gain Stability allowed. With the present system breakdown, the assignment for the gain stability of the cryogenic amplifier was specified in terms of Allan Variance in the time interval from 0.1 to 1.0 seconds. (sec) 1 1 . 0 10 2 ) ( 8 2 ≤ ≤ ⋅ ≤ − τ τ σ [1] A recent proposal from System Engineering (September 2006) sets different limits. The new values would be:     ≤ < ⋅ ≤ ≤ ≤ ⋅ ≤ − − (sec) 300 100 10 2 ) ( (sec) 100 05 . 0 10 2 ) ( 9 2 8 2